A prior-art method of embedding a watermark in a motion image signal is disclosed in International Patent Application WO-A-99/45705. In this prior-art method, a two-dimensional sequence of watermark samples is completely, and even a plurality of times, embedded in an image of a video signal.
Watermark embedding is an important aspect of copy protection strategies. Although most copy protection schemes deal with protection of electronically distributed contents (broadcasts, storage media), copy protection is also desired for movies being shown in theaters. Illegal copying in the cinema by means of a handheld video camera is already common practice. The quality is usually very low, but the economical impact of illegal VHS tapes, CD-Videos and DVDs can be enormous.
In the coming years, the digital cinema format, 1920×1080×24×36 (pixels/line×lines/frame×frames/s×bits/pixel), will be introduced in the theaters. By introducing this very high-quality digital format, the threat of illegal copying by handheld video cameras will be even larger. For this reason, cinema owners are obliged to prevent the presence of video cameras on their premises. Not abiding by this rule may be sanctioned with a ban on the future availability of content. In view thereof, it is envisioned to add a watermark during show time. The watermark is to identify the cinema, the presentation time, operator, etc.
Most watermark schemes, including the one mentioned in the opening paragraph, are sensitive to alignment errors at detection time. Solutions have been published to either insert the watermark in a domain that is invariant for a certain class of geometrical transforms, or to find back the alignment during detection. A disadvantage of these methods is that they can generally only cope with a limited number of geometrical transformations. Furthermore, these methods usually decrease the robustness to other attacks.
The requirements for the digital cinema watermark, similarly as for any other watermarking scheme, are: (i) robustness, (ii) imperceptibility and (iii) a low false positive rate. Achieving sufficient robustness is the most challenging requirement. The handheld camera will not only seriously degrade the video by filtering (the optical path from the screen to the camera, transfer to tape etc.) but also seriously geometrically distort the video (shifting, scaling, rotation, shearing, changes in perspective, etc.). In addition, these geometrical distortions can change from frame to frame.